The present invention relates to a method for the preparation of novel inorganic fibers or, more particularly, inorganic fibers containing silicon, carbon, boron and nitrogen as the constituent elements and having excellent heat resistance and physical and mechanical properties.
As is known, fibers mainly composed of silicon carbide have been developed in recent years with an intended application as a reinforcing material in several high-performance composite materials such as FRM (fiber-reinforced metal) and FRC (fiber-reinforced ceramic). The silicon carbide-based fibers are highlighted in such an application in place of carbon fibers by virtue of not only the more excellent heat resistance and oxidation resistance but also the higher affinity and wettability with the matrix material such as metals than carbon fibers. A method for the preparation of silicon carbide-based fibers is disclosed, for example, in Japanese Patent Publication Nos. 57-53892 and 57-56566 according to which a polycarbosilane having a main chain composed of silicon and carbon is first synthesized and it is spun into a fibrous form followed by infusibilization and calcination to be converted into silicon carbide. Further, several attempts have been made with an object to improve the mechanical properties of the silicon carbide-based fibers derived from polycarbosilanes by introducing other kinds of elements thereinto including the introduction of a borosiloxane disclosed in Japanese Patent Kokai No. 54-82435, titanoalkoxide disclosed in Japanese Patent Publication No. 58-5286 and zirconoalkoxide disclosed in Japanese Patent Kokai No. 57-106718.
Several problems are, however, unavoidable in the above described process for the preparation of silicon carbide-based fibers via polycarbosilanes. For example, the polycarbosilane compound must be synthesized by a reaction under a high pressure at a high temperature taking a long time. In addition, the yield of the desired polycarbosilane is disadvantageously low with low productivity even by setting aside the problem in connection with the apparatus used for the reaction. The introduction of the above mentioned different elements such as boron, titanium and zirconium into the polycarbosilane is also not free from problems and disadvantages in the properties of the resultant silicon carbide-based fibers because these heteroatoms are always bonded to the silicon atoms through an intermediate oxygen linkage such as -Si-O-B-, -Si-O-Ti- and -Si-O-Zr- so that the silicon carbide-based fibers contain an increasing amount of oxide as the amount of the introduced heteroatoms is increased. Further, the silicon carbide-based fibers prepared in this manner are usually composed of aggregates of extremely fine crystallites or grains of silicon carbide having a particle size of about 2 nm and heating of the fibers at 1300.degree. C. or higher causes growth of the crystallites or grains resulting in decrease of the mechanical strength of the fibers.